1. Field of the Invention
The present invention relates to a precursor composition for a polyimide (PI). The present invention also relates to the use of the precursor composition in the preparation of polyimides.
2. Description of the Prior Art
Due to their superior thermal stability and excellent mechanical, electrical, and chemical properties, polyimides have become the top choice among high performance polymeric materials. Rising semiconductor standards have highlighted the limitations of conventional inorganic materials and accentuated the ability of polyimides to resolve aspects of these shortcomings. Since their introduction by the E.I. Du Pont Company, polyimides have become widely used in a variety of applications.
In the semiconductor industry, polyimides have been extensively used in passivation coatings, stress buffer coatings, α-particle barriers, dry-etch masks, microelectromechanical and interlayered insulation films. Still more uses are being developed. Polyimides are primarily used as a protective coating for integrated circuit elements because the polyimide materials can pass reliability testing of integrated circuit elements. Outside of the integrated circuit industry, polyimides are also used in electronic packaging, enamelled wires, printed circuit boards, sensing elements, separating films, and structural materials.
Polyimides are typically synthesized in a two-stage polymerization and condensation reaction. In the first stage, a diamine monomer is normally dissolved in a polar aprotic solvent, such as N-methylpyrrolidone (NMP), N,N-dimethylacetamide (DMAC), dimethylformamide (DMF), or dimethyl sulfoxide (DMSO). A molar equivalent of a dianhydride monomer is then added. Afterwards, the condensation reaction is conducted at low or room temperature to form a precursor for the polyimide, i.e., poly(amic acid) (PAA).
In the second stage, thermal or chemical imidization is carried out to achieve a condensation, dehydration, and cyclization reaction so as to convert the poly(amic acid) into a polyimide.
Under current practice, the reaction scheme for preparing polyimides can be summarized by the following diagram:

In the above preparation method, if the molecular weight of the poly(amic acid) obtained in the first stage does not reach a certain standard (i.e., it is overly low), a polyimide film having good physical properties cannot be obtained after imidization. However, if the molecular weight of the poly(amic acid) obtained in the first stage is overly high, the PAA will be too viscous to be operable. In addition, poor leveling easily occurs in the coating step. For example, spin coating may produce a convex middle and thick edges. Moreover, if the poly(amic acid) is overly high in molecular weight, an extremely strong internal stress is produced due to the interaction between molecules and the shortening of molecular chains in the imidization of the second stage. The strong internal stress causes the coated substrate to bend and deform. To address these problems, various studies have explored the relationship between the gradient heating curve control during the imidization of the second stage and the internal stress. Various approaches to decreasing the internal stress have been developed as well. Regardless of approach, the primary cause of the problems of leveling and internal stress is overly high molecular weight of the poly(amic acid) obtained in the first stage. In other words, if the molecular weight of the poly(amic acid) can be adequately controlled, a polyimide film with excellent physical properties can be achieved.
TW Patent Application No. 095141664 discloses a precursor composition for polyimides comprising an amic acid oligomer having two terminal amino groups and a dianhydride derivative with both ester (—C(O)OR) and carboxy (—C(O)OH) terminal groups which can maintain a meta-stable status with the amic acid oligomer and thus will not react with the two terminal amino groups of the amic acid oligomer at room temperature. In addition, since the amic acid oligomer has a lower molecular weight, the precursor composition has excellent operability and the resultant polyimides exhibit excellent thermal, mechanical, and stretching properties. However, this precursor composition can only be subjected to thermal imidization. In thermal imidization, 100% imidization generally requires heating at 250° C. to 350° C. for several hours, a process which is lengthy and liable to cause industrial safety problems. Moreover, in products with side chains of a low bonding energy, high temperature will cause the side chains to break before cyclization.
The present invention discloses a special synthesis method of subjecting a specific polymide precursor composition to low temperature dehydration and cyclization, with consideration of operability, to obtain polyimides having desirable physical properties meeting the demands of industry.